Measuring instruments of the above type have been known for some time; reference may be made purely by way of example to DE 198 19 492 A1, which discloses a measuring instrument. This measuring instrument serves a purpose of high precision measurement of the coordinates of patterns on substrates, for example masks, wafers, flat screens, vacuum-deposited patterns, semiconductor substrates, illumination masks or optical data media, in particular, however for transparent substrates. The coordinates are determined exactly to a few nanometers relative to a reference point. In this optical inspection technique, complex patterns of objects on flat substrates are inspected by image field. When objects are inspected by image field, the object is usually moved relative to the imaging optics with the aid of positioning means—in the form of positioning or measuring stages, for example—such that different areas of the object can also be detected.
A position evaluation device usually has means with which an object pattern can be detected and/or classified, with the aid of which a further object pattern at another point of the object—if appropriate, positioned with the positioning means at another position relative to the imaging optics—can likewise be detected and/or classified, and with which the position of the object patterns relative to one another can be determined. These means can, for example, be a CCD camera, a computer and an appropriate analysis and evaluation program, it being possible for the CCD camera to detect the image field of the object at a position and feed it to a computer in digitized form. An evaluation program running on the computer carries out the detection and/or classification of the object patterns with the aid of the digitized image data and object information possibly prescribed, as well as determining their distances.
Since the objects to be detected increasingly have smaller patterns, it is necessary to raise the optical resolution of the measuring instrument. This can be achieved by increasing the numerical aperture of the imaging optics and/or by reducing the wavelength of the light used for detection. The nominal resolution of an imaging optics can thereby be increased, above all for point objects, such that the diameter of the central diffraction maximum or of the classic diffraction disk can be reduced in the object illumination. However, both approaches to the solution encounter limits in principle that render a further increase in resolution very expensive. However, precision measurements of line patterns in a multiple resolution spacing are problematical, in particular, since the secondary maxima of the classic diffraction disk likewise impair the measurement of points of patterns which, starting from an initial point, are certainly not located at the spacing of the first diffraction minimum. The secondary maxima of the classic diffraction disk have a mutual spacing that far exceeds the resolving power of the imaging optics. The smaller the measuring error required, the greater must be the spacing of the measured patterns from the neighboring patterns. This results in a reduction of the effective resolution of the patterns by the measuring instrument.